CHEMICAL BONDS IN NUCLEIC ACIDS 



427 



that digestion of ribonucleic acid with ribonuclease, followed by treat- 

 ment with alkali, yielded only the b isomers of the pyrimidine nucleotides, 

 together with a mixture of the a and b isomers of the purine nucleotides. 

 The processes involved are shown in the annexed formulas in which Py = 

 uracil or cytosine residue and R = remainder of polynucleotide chain; 

 where R = H the final products of ribonuclease action are the pyrimidine 

 b mononucleotides which are unaffected by treatment with alkali. 



Ribonucleic acid 



Pyrimidine 6 nucleotides and 

 purine a+ b nucleotides 



CHj-OR 



The evidence so far presented in this section regarding ribonuclease ac- 

 tion may now be summarized. The initial reaction of the enzyme evidently 

 occurs specifically at pyrimidine nucleotide sites in the ribonucleic acid 

 molecule. This specificity is discussed in more detail later, but it is supported 

 by the observed faihire of ribonuclease to attack the cyclic 2 ',3 '-phos- 

 phates of adenosine and guanosine,*^'^"^ although it readily attacks their 

 uridine and cytidine analogues. Subject to this specificity, and to the pro- 

 duction of only the b isomers of pyrimidine mononucleotides, ribonuclease 

 digestion follows a course akin to that of alkaline hydrolysis. The course 

 of ribonuclease hydrolysis and the various degradations described in this 

 section can be conveniently represented in the appended scheme using the 

 earlier postulated 3',5'-linked polynucleotide to represent ribonucleic acid 

 (in this scheme Py = uracil or cytosine residue, Pu = adenine or guanine 

 residue, and C2' — C3. — €5- is again used as an abbreviated form of the sugar 

 residue). In this representation of ribonuclease action, the pyrimidine 

 mononucleotide fraction arises from sites in the nucleic acid where two or 

 more pyrimidine nucleotide residues are adjacent, a dinucleotide from sites 

 where a purine nucleotide is flanked on either side by a pyrimidine nucleo- 

 tide, and larger fragments from positions where several purine nucleotides 

 occur consecutively."'' '"'* The production of another type of fragment by 

 ribonuclease action, viz., the so-called "core,"^''" is not substantiated by 

 recent work.^^"^ It should be pointed out that although the reactions are 



1'^ R. Markham and J. D. Smith, Biochem. J. 52, 565 (1952). 



